The present invention relates generally to methods for forming phase-shifting photomasks used to fabricate semiconductor structures, and specifically a method for forming an improved attenuated phase-shifting photomask by optimizing the first of two electron beam dosages.
As integrated circuits (IC) become smaller and smaller through miniaturization, photolithographyxe2x80x94the process that sets the surface dimensions of the various parts of the devices and circuits of the integrated circuitsxe2x80x94has advanced to 0.35 microns and below. Thus new technologies are required to increase the resolution of the imaging lens, i.e. the ability to apply surface patterns as close to the design requirements as possible. Several techniques have developed to accomplish this requirement.
Phase shifting photomasks (PSM) permit the use of interference between out of phase light waves to reduce the spatial frequency of a given object, to enhance its edge contrast, or both. By applying a patterned layer of transmitting material that delay the light waves 180xc2x0 out of phase at critical locations in a design, the 180xc2x0 out of phase light waves cancel out the light waves passing through an adjacent clear area edge producing a more sharply defined interface.
Attenuated phase-shifting photomasks (APSM) use an attenuatorxe2x80x94a metallic-like absorbing film such as molybdenum silicide oxynitride (MoSiON) or chromium oxynitride (CrON)xe2x80x94that allows 5-15% light transmittance. The partial light transmittance through the attenuator causes production of phase shifted light.
The electron-beam (E-beam) double exposure method of halftone APSMs causes overexposure for large exposed areas, especially in the letters"" pattern and the frame pattern. Letters are used to identify individual photomasks so they may be traced for quality control measures. This causes chromium (Cr) and attenuator material loss in the subsequent steps of the double exposure method of halftone APSM formation leading to, for example, many pinholes and nuisance defects that are then discovered during photomask inspection.
U.S. Pat. No. 5,783,337 to Tzu et al. describes a process for fabricating an attenuated phase-shifting photomask (APSM). The resist layer of the photoresist mask blank is divided into three areas: a main pattern area; a secondary pattern area; and a border area. The photoresist mask blank is exposed to electron-beam (E-beam) energy wherein the main pattern area receives a first dose of E-beam energy, the secondary pattern area received a second dose of E-beam energy that is less than the main pattern area""s first dose of E-beam energy, and the border area is not exposed to the E-beam dose. The photoresist layer is then developed, the photoresist layer overlying the main pattern area is removed to expose the underlying Cr layer which is then etched to expose the underlying attenuator layer. The exposed attenuator layer is in turn etched to expose the underlying substrate. The photoresist layer overlying the secondary pattern layer is then etched away exposing the Cr layer which is etched to expose the underlying attenuator layer. Lastly, the photoresist layer overlying the border area is stripped away leaving a patterned phase-shifting layer in the main pattern area and a Cr layer in the border area that prevents light leakage from around the border.
U.S. Pat. No. 5,723,235 to Tsudaka et al. describes a method of producing a photomask, exposing the photomask produced, and a method of manufacturing a semiconductor device using the produced photomask. An optimization method is used for exposing resist to produce the photomask.
U.S. Pat. No. 5,532,090 to Borodovsky describes a method and apparatus for forming openings in a photosensitive layer by first exposing an unpatterned photosensitive layer to a first mask having an opening pattern with dimensions within tight (for a given technology generation) process tolerances. Next, prior to development, the photosensitive layer is exposed to a second mask having a grid of clear spaces surrounding the opening pattern. The patterns and exposure doses of the first and second reticule are designed to produce a combined intensity profile and corresponding latent image in the photoresist layer that results in an opening/via pattern of reduced dimension upon resist development.
U.S. Pat. No. 5,888,678 to Tzu et al. describes an attenuated phase-shifting mask and a method of making same that has a rim type attenuating phase-shifting mask (APSM) pattern, for formation of small contact holes, in a second region of a transparent mask substrate and a binary mask pattern, for formation of larger holes, in a first region of the same transparent mask substrate. The formation of the rim type APSM pattern and the binary mask pattern on the same transparent mask substrate avoids the problems due to side lobe effects and increases throughput and decreases fabrication costs of integrated circuit wafers.
U.S. Pat. No. 5,853,923 to Tzu describes a method of forming a rim type APSM which requires only one resist layer and one resist developing step using a single developing solution. The resist layer is exposed to a first pattern using a first exposure dose sufficient to expose the first pattern in the entire thickness of the resist layer. The resist layer is then exposed to a second pattern using a second exposure dose sufficient only to expose the second pattern in a top portion of the resist layer.
U.S. Pat. No. 5,804,339 to Kim describes methods of fabricating a photomask including a correction exposure, that is less than a first exposure dosage, utilizing a correction exposure mask.
Accordingly, it is an object of the present invention to optimize the first E-beam dosing of a double E-beam exposure method by a segmentation method to form attenuated phase-shifting photomasks (APSM).
Another object of the present invention is to optimize the first E-beam dosing of a double E-beam exposure method by a segmentation method to reach uniform thickness of exposed photoresist of halftone APSMs.
A further object of the present invention is to optimize the first E-beam dosing of a double E-beam exposure method by a segmentation method to form attenuated phase-shifting photomasks to avoid overexposure for large exposed areas.
Yet another object of the present invention is to optimize the first E-beam dosing of a double E-beam exposure method by a segmentation method in forming attenuated phase-shifting photomasks to reduce pinholes and nuisance defects found during APSM inspection.
Other objects will appear hereinafter.
It has now been discovered that the above and other objects of the present invention may be accomplished in the following manner. Specifically, the first E-beam dosage of a double E-beam exposure method is optimized by a segmentation method to reach uniform thickness of the exposed photoresistor of halftone attenuated phase-shifting photomasks (APSM). A photomask blank is provided. The photomask blank having an upper resist layer overlying a chromium layer, the chromium layer overlying a phase-shifting layer, and the phase-shifting layer over a substrate. The photomask blank having a low pattern density area with a pattern density less than 0.25, a middle pattern density area with a pattern density from about 0.25 to 0.70, and a high pattern density area with a pattern density between about 0.70 and 1.00. The photomask blank is exposed to a first E-beam energy in a single step wherein the low pattern density area is exposed to the first E-beam energy adjusted by a first dosage factor, the middle pattern density area is exposed to the first E-beam energy adjusted by a second dosage factor, and the high pattern density area is exposed to the first E-beam energy adjusted by a third dosage factor. The photoresist blank is exposed to a second E-beam energy in a single step wherein the low and middle density areas are equally exposed to the second E-beam energy. The exposed resist layer is exposed wherein portions of the resist is removed to expose the chromium layer. The exposed chromium layer is etched to expose the underlying phase-shifting layer. The exposed phase-shifting layer is etched to expose the substrate. The resist overlying the chromium layer is etched away to complete fabrication of the attenuating phase-shifting photomask.